Conventionally, power amplifier concepts operate the power amplifiers with fixed input and output loads. These loads are usually chosen such that the gain, the output power or the efficiency is optimized depending on the need of the specific application. In particular regarding mobile communication systems, such an application may be a battery-powered amplifier in a user equipment or mobile station or may be an amplifier for a base station.
In case of battery powered amplifier the load usually is chosen such that the efficiency is optimized, wherein in case of an amplifier for a base station the optimization for a maximum output power is needed.
Normally it is not possible to get an optimum for all the above-mentioned three parameters at the same time as each of these parameters is requiring at least a slightly different input/output matching network.
For signals with a peak power to average power ratio of one, such as a frequency modulated (FM) or phase modulated (PM) signal or a GMSK (Gaussian Minimum Shift Keying) signal used in a GSM (Global System for Mobile communication) system, this conventional design concept usually works well as the power amplifier operates all the time near its maximum output power, where the amplifier yields a high efficiency so that the loads which have to be chosen for an optimum output power and an optimum efficiency normally are not very different.
For the signals however, with high peak to average power ratios, the power amplifier has to be operated most of the time at an output power level that is some dB below the maximum output power, such as for example 8 to 10 dB below the peak power for a W-CDMA (Wideband-Code Division Multiple Access) signal in the downlink path. For the average power level the transistor efficiency is only a small part, for example one fifth, of that efficiency at maximum output power. Hence for signals with high peak to average ratios, most of the consumed DC power is dissipated in heat and only a small part of this DC power is converted to the wished RF power. As a consequence, the high DC power increases operating costs and the large amount of dissipated heat increases costs for the amplifier cooling equipment including operating costs for active cooling systems, such as air conditioning, and material costs for bigger heat-sinks and better heat conducting materials.
Consequently, from a standpoint of costs and environmental friendly design, it is advantageous to have a power amplifier that has an efficiency that is as high as possible for signals with high peak to average ratios.
Known concepts for enhancing the efficiency of a power amplifier under variable envelope excitation are in particular the concept according to the Doherty amplifier, to the Chireix's outphasing amplifier, to the Envelope Elimination and Restoration (EER), as invented by Kahn, and to the Bias adaptation.
Regarding the Doherty amplifier in more detail, two amplifiers with a special combining network are used. The amplifiers are called main amplifier and auxiliary amplifier. For output power levels close to the combined maximum output power of both transistors, both amplifiers are active. As the output power is reduced, the auxiliary amplifier shuts down at a certain level and generates no more RF power, whereas the main amplifier is still active. Such a shut down is typically at 6 dB below the maximum combined output power. For this power back-off condition the main amplifier is near its maximum output power and thus the efficiency is almost at its optimum. However, one of the most drawbacks of this concept is, that two amplifiers with a special controlling and combining network are needed.
Regarding the Chireix's outphasing amplifier, the amplitude modulation of an input signal that causes the high peak to average power ratio is converted to a phase modulation. This converting process is done at baseband frequencies usually by employing digital processing, wherein the converting process generates two phase-modulated signals that have to be processed in parallel up to the output of the amplifier unit. As these two signals contain only phase modulation, highly non-linear amplifiers can be used that yield good efficiency. The original signal with its original phase modulation content and original amplitude modulation content is reconstructed at the output of both non-linear amplifiers with help of a special combining network. As a disadvantageous consequence, two complete amplifier chains from the baseband up to the output of the amplifier unit are necessary and the combining network at the output is difficult to realize.
By using the known concept of envelope elimination and restoration (EER), the amplitude modulation of the amplifier input signal is sensed via an envelope detector. Then the amplitude modulation is taken from the input signal by passing it through a limiter. The phase modulation of the input signal is preserved by this process. As the input signal to the amplifier then contains only phase modulation, a high efficient non-linear amplifier can be used. The amplitude modulation of the original signal is restored at the output of the amplifier by changing the amplifiers supply voltage according to the envelope detector output voltage. The efficiency enhancement is based on the fact that an amplifier efficiency is best when the amplifier supply voltage is just as high to accommodate the necessary voltage swing at the amplifier load.
However, the controlling of the supply voltage or current for the amplifier is difficult to achieve because of the speed of the necessary voltage and current changes. Furthermore, the efficiency of the necessary DC/DC converter worsens the efficiency budget of the whole amplifier unit.
Thus the drawbacks of the known EER technique are lying mainly in the fact, that even the most recent DC/DC converters are not able to provide the fast changes in the amplifier supply voltage and/or current needed to reconstruct the amplitude of the original signal at the amplifier unit output, which holds true especially for broadband signals like a UMTS (Universal Mobile Telecommunication System) signal with high power amplifiers.
Finally, the bias adaptation concept is in principle the same as the envelope elimination and restoration, but a conventional linear RF-amplifier is used instead of a saturated amplifier. Accordingly, the amplitude content is not removed from the input signal.